We report the rare occurrence of essential thrombocythemia (ET) in two sisters. In one patient, the clinical phenotype of the disease evolved from ET to polycythemia vera (PV) after 4 years of follow-up. Clonal hematopoiesis was established in both cases by X-chromosomal inactivation analysis using a DNA polymorphism of the phosphoglycerate- kinase (PGK) gene. Cell separation studies suggested a common ancestor for granulocytes, monocytes, and T lymphocytes in one patient; however, in her sister, monoclonality could only be demonstrated convincingly for the granulocyte fraction. Our data indicate that ET may originate from heterogenous stem cell levels.